Microbial pathogens such as Mycobacterium tuberculosis and Francisella tularensis are attractive agents for bioterrorism due to their infectious nature and ability to cause severe pathogenesis. As such, understanding the mechanisms by which the mammalian immune system mounts an effective response to these pathogens is of utmost importance. Recently, there has been tremendous progress in identifying the transcription factors that regulate both the development of Th1 cells and their expression of interferon-gamma (IFN-gamma), although in many cases the exact mechanisms by which these signaling molecules contribute to the generation of a Th1 cell response remain to be determined. Previously, we demonstrated that signal transducers and activators of transcription (STAT) 4 is critical for the normal development of IFN-gamma producing Th1 cells, and more recently we have identified several novel molecules that appear to regulate Stat4 expression and/or function. Our overall goal in this grant application is to gain a further understanding of the role of Stat4 in regulating the immune response to pathogens. We have already generated several strains of mice in which the levels of Star4, and thus IFN-gamma, is altered, and we propose creating new strains of mice in which Stat4 expression can be regulated in a spatial or temporal manner. The experiments proposed will investigate the outcome of infection with microbial pathogens in mice with altered Stat4 expression and/or function, examine the mechanisms that regulate Stat4 expression, and identify interacting proteins that regulate Stat4 function. Taken together, the experiments detailed in this grant application are aimed at elucidating the molecular mechanisms governing the generation of an effective cell-mediated immune response to pathogens, in general, and on the role of Stat4 in that process in particular. It is hoped that this information will ultimately lead to new strategies for manipulating the immune response, be that at the preventative phase as in vaccine development, in the effector phase of pathogen eradication, or in later stages of the immune response such as the maintenance of long-term memory.